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When the heating fails, it’s often not the boiler — it’s the system around it. Sludge, trapped air, a stuck valve or lost pressure account for many Eltham heating faults. This page is about finding where the heat is actually being lost, and who fixes it. Skip to verified engineers ↓
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Smell gas or fumes? Leave and call National Gas on 0800 111 999 first — otherwise, here’s how Eltham heating faults get diagnosed and fixed.
Contact verified heating engineers in Eltham ↓
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Coverage: verified heating engineers across Eltham and the wider SE9 area — the town centre, Well Hall and the Progress Estate, Eltham Park, Eltham Green, Mottingham, New Eltham and Avery Hill.
What this covers: cold radiators, lost pressure, sludge and air, stuck valves and pumps, expansion-vessel and system-clean work — the wet side of central heating.
Not this page? A boiler that won’t fire or shows a fault code is Boiler Repair; a new or replacement boiler is Boiler Installation; a hidden leak is Leak Detection; a gas smell is an emergency — call National Gas on 0800 111 999.
Costs: indicative ranges are under What it costs below — editorial estimates only.
Availability: each engineer sets their own hours, shown on their profile.
Jump to: Common faults · Cold radiators · A proper first visit · Losing pressure · The gas-safety line · Hard water & sludge · Council, lease & rent · Costs · How faults develop · Why verified · FAQ
Common central heating faults in Eltham homes
Most Eltham heating is served by individual boilers — combis in smaller and converted homes, and system or heat-only boilers with cylinders in the larger interwar semis around Well Hall, Eltham Park and Avery Hill. One thing to check first: if you’re in a flat, a conversion or a newer block, “no heat” can be a communal or landlord-owned heating system rather than a private repair — confirm which you have before booking an engineer, so you’re not paying for a fault that isn’t yours to fix.
For individual-boiler homes, the fault patterns are consistent:
- No heat or no hot water — could be the boiler, but just as often a failed pump, diverter valve, thermostat, timer or lost pressure.
- Some radiators cold, others hot — a distribution problem: sludge, air, or a system that’s never been balanced.
- A radiator cold at the bottom but warm at the top — the classic sludge signature.
- A radiator cold at the top but warm at the bottom — trapped air, usually cured by bleeding.
- A radiator dead cold with the others hot — often a seized thermostatic radiator valve (TRV) or a failed motorised (zone) valve, not the radiator itself.
- Banging or rumbling (“kettling”) — scale or sludge on the heat exchanger causing localised boiling.
- Pressure dropping on a sealed system — water is going somewhere, and that matters (see below).
Knowing which pattern you have lets you brief an engineer precisely — and tells you whether it’s a five-minute bleed or a system that needs cleaning.
Cold radiators: sludge, air, or balancing?
Three different faults look similar and have different fixes:
Air (cold at the top). Air rises to the top of a radiator and stops hot water filling it. Bleeding the radiator releases it. If air keeps returning, something is drawing it in — a failing auto air vent, a pump pushing too hard, or repeated re-pressurising masking a leak.
Sludge (cold at the bottom). Magnetite sludge — black iron-oxide debris — is heavier than water and settles in the bottom of radiators and in low points, blocking flow. Bleeding won’t help. The fix is a system clean, and the choice between two methods matters:
- A chemical flush — dosing a cleaner, circulating it, then draining and refilling with fresh inhibitor — is enough for lighter contamination and early cold spots. It’s the proportionate first step on a system that’s simply been neglected.
- A power flush — machine-driven high-flow flushing, radiator by radiator — is for heavy sludge with several cold radiators, a sluggish system or a boiler that keeps tripping on debris. It’s more disruptive and more expensive, which is exactly why it shouldn’t be sold before the fault is diagnosed.
Balancing (whole rooms consistently under-heating). If the radiators nearest the boiler run hot while far rooms stay cool, the system may simply never have been balanced — the lockshield valves set so flow is shared evenly. Balancing is an adjustment, not a part. In an older Well Hall or Progress Estate home with original circuits and long pipe runs, a never-balanced system is a common and cheap-to-fix cause of cold back bedrooms.
A stuck TRV or a failed motorised valve produces a single dead-cold radiator or a whole zone that won’t heat — both are component swaps, not a reason to flush the system.
A proper first visit: what a good diagnosis looks like
The difference between a fair heating repair and an expensive guess is the diagnosis. Before recommending anything — and certainly before quoting a power flush — a good engineer works through a logical sequence:
- Confirm demand — is the thermostat and programmer actually calling for heat?
- Check the boiler — status, any fault code, that it’s firing.
- Read the pressure — on a sealed system, is it holding around 1–1.5 bar cold?
- Test the moving parts — pump running, diverter and motorised valves operating.
- Read the radiator pattern — cold tops, cold bottoms, or whole cold zones each point somewhere different.
- Assess the system water — its condition decides whether it’s a clean or a component fault.
That last step has a simple tell: heavy black debris on a magnetic filter’s magnet is direct evidence of magnetite corrosion, and it points to cleaning and water treatment rather than repeatedly replacing parts that keep failing in dirty water. If a system has no filter, that’s often the first thing worth fitting.
Losing pressure: where the water goes
A sealed (pressurised) system should hold a steady pressure, typically around 1–1.5 bar cold. If yours keeps dropping and you keep topping it up, the water is escaping somewhere — and repeatedly topping up is a warning sign, not a fix, because each top-up introduces fresh oxygenated water that accelerates corrosion.
Two common causes: a leak somewhere in the pipework, radiators or boiler (often a slow one — this is where our leak detection service comes in), or a failed expansion vessel that can no longer absorb the expansion of heated water, pushing pressure out through the safety valve. A dripping external pressure-relief pipe is a classic sign of the latter. In newer New Eltham stock, where sealed pressurised systems are the norm, a slow pressure drop is more often the expansion vessel or fill loop than a burst — which is why the diagnosis, not just a top-up, is the job.
The gas-safety line every heating repair respects
Central heating work divides into two sides. The “wet” side — radiators, valves, pumps, pipework — can be done by a competent plumber. But work on the boiler itself and the gas supply is different: the HSE is clear that anyone working on gas appliances in domestic premises must be Gas Safe registered and competent in that area, and that it is illegal to use a gas appliance you suspect is unsafe — turn it off and don’t touch it until a Gas Safe registered engineer has checked it.
If you ever smell gas or suspect a leak, the HSE’s instruction is to call 0800 111 999, open doors and windows, and shut off the gas at the meter control valve if you know where it is — that is not a plumber call in the moment. Yellow or sooty flames, staining around the boiler, or a smell of fumes point to combustion problems and mean the appliance should be shut down until an engineer assesses it.
Hard water, sludge and why Eltham systems clog
Eltham is in Thames Water’s supply area, and Thames Water states all the water in its region is hard because it passes through chalky limestone. Two different problems then build up inside a heating system, and it’s worth separating them:
- Limescale is the hard-water problem — dissolved minerals depositing on the hottest surface, the heat exchanger, where they cause kettling and lost efficiency.
- Magnetite sludge is a corrosion problem — the reaction of the steel in radiators with water and dissolved oxygen, producing black iron oxide. Hard-water scaling and repeated fresh-water top-ups make it worse, but the corrosion is the root cause, which is why water treatment matters as much as descaling.
The defence covers both: a clean system, a corrosion inhibitor at the correct dose, and a magnetic filter to trap circulating iron oxide before it settles. On an older, never-treated Eltham system, this can be one of the most cost-effective repairs — it restores flow, quietens kettling and protects the boiler. Our London hard water guide explains the mechanism in more detail.
Council tenants, leaseholders and renters: the heating route
Who fixes your heating depends on how you hold your Eltham home.
Council tenants report to Royal Greenwich, not a private plumber. Loss of heating and hot water is treated as an urgent repair, and the council’s timescale is season-dependent: it aims to restore heating within one working day between 1 November and 30 April, and within three working days between 1 May and 31 October. Report urgent problems on 020 8921 8900 in office hours or 020 8854 8888 at all other times. For a gas problem specifically, the council routes tenants to Royal Greenwich Building Services on 0800 317 715 or K&T Heating Services on 0800 0434 726.
Leaseholders in a Royal Greenwich block are responsible for their own internal systems — the council’s repair table makes leaseholders responsible for heating and hot water systems inside the home — so you arrange and pay for your own heating repair.
Private tenants contact the landlord or agent first; where the fault is a serious hazard such as no hot water, Royal Greenwich’s Residential Services team can step in on 020 8921 8157, and for non-urgent issues the council’s rule is to give the landlord 14 days to respond before escalating.
What central heating repair costs in Eltham — 2026
Editorial estimate only — not regulated or official pricing, and no substitute for a written quote. Parts prices vary by make; anything on the boiler or gas side must be done by a Gas Safe registered engineer.
| Repair | Typical range |
|---|---|
| Diagnostic call-out / fault-find | £70–£120 |
| Bleed and balance radiators (visit) | £70–£140 |
| Replace radiator (like-for-like) | £150–£350 |
| Replace TRV or lockshield valve | £90–£180 |
| Replace motorised (zone) valve | £180–£320 |
| Replace circulating pump | £180–£350 |
| Replace diverter valve | £250–£450 |
| Re-pressurise / replace expansion vessel | £120–£350 |
| Replace PCB (control board) | £250–£500+ |
| Auto air vent / fill-loop fault | £90–£200 |
| Chemical flush + inhibitor | £150–£350 |
| Power flush (typical 8–10 radiators) | £400–£800 |
| Fit magnetic system filter | £120–£250 |
| Replace thermostat / programmer | £120–£300 |
Ask for the pricing structure — call-out plus labour, or an inclusive rate — before the engineer attends, and get the fault diagnosed before authorising a power flush. Eltham is inside the London Ultra Low Emission Zone, so a non-compliant van may carry the £12.50 daily charge — check the current position.
How heating faults actually develop
Understanding the mechanism helps you see why some repairs recur and others last.
Magnetite corrosion. Steel radiators, water and dissolved oxygen react to form iron oxides — magnetite, the black sludge at the heart of most heating faults. It circulates until it settles in low-flow areas: the bottom of radiators (cold spots), the pump (premature failure) and the heat exchanger (restricted flow and kettling). Every time a system is drained, topped up or leaks and is refilled, fresh oxygenated water feeds the reaction — which is why chasing a pressure leak matters beyond the pressure itself.
Kettling. On a scaled or sludged heat exchanger, water in contact with the hot metal can locally overheat and flash to steam, then collapse — the rumbling, kettle-like noise. It signals reduced heat transfer and a hard-working boiler, and it’s usually a cleanliness-and-water-treatment problem, not a boiler that needs replacing.
Air ingress. Persistent air isn’t always innocent. It can enter through a failing automatic air vent, be drawn in at a pump running against restricted flow, or accompany the fresh water of a slow leak. Recurrent bleeding is a symptom to investigate, not a routine.
Sealed-system pressure. A pressurised system relies on an expansion vessel — a sealed chamber with a gas cushion — to absorb the expansion of water as it heats. When the vessel loses its charge, expanding water has nowhere to go and forces pressure out through the relief valve, which then weeps. Re-charging or replacing the vessel fixes the cause; topping up the pressure only delays the next discharge.
Why verified engineers in Eltham
Heating repair is where a clear diagnosis saves you money — and where the wrong call means paying for parts you didn’t need. Every engineer listed here is checked before they appear: identity, insurance, trading presence and Gas Safe registration where gas work is involved, and re-checked annually. Plumbers pay a flat monthly listing fee, so there’s no customer-side introduction fee and no pay-to-play ranking. Workmanship-guarantee badges show whether the repair is backed for 1, 3, 6 or 12 months — and you should still confirm current Gas Safe registration for any work on the boiler or gas side. On a period or conservation-area home like the Progress Estate, ask that any visible pipework alterations are routed sympathetically. Listing checks are administrative and don’t replace your own checks or guarantee workmanship; see how we verify plumbers.
Frequently asked questions
Cold at the bottom is sludge, not air, so bleeding won’t help.
That needs a system clean — chemical flush, or power flush if heavy — plus fresh inhibitor.
Cold at the top is air, which bleeding fixes.
A single dead-cold radiator with the rest hot is more likely a stuck TRV or motorised valve.
It means water is escaping.
That could be a leak or a failed expansion vessel forcing pressure out through the relief valve.
Repeated topping-up accelerates corrosion, so it’s worth diagnosing rather than living with.
A dripping outside overflow pipe points to the expansion vessel.
It depends how sludged the system is.
Light contamination often responds to a chemical flush and inhibitor.
Heavy sludge with several cold radiators usually needs a power flush.
A magnetic filter then keeps new sludge from recirculating.
A good engineer diagnoses — including checking the system water — before recommending the more expensive option.
One quick tell is heavy black debris on a magnetic filter’s magnet.
That’s evidence of magnetite corrosion.
If parts keep failing in a dirty system, cleaning and water treatment usually fix more than another replacement will.
Royal Greenwich, not a private plumber.
Between 1 November and 30 April the council aims to restore heating within one working day.
Call 020 8921 8900 in office hours.
Call 020 8854 8888 out of hours.
For a gas fault, call Building Services on 0800 317 715.
Not always.
Some flats and blocks run on communal or landlord-owned heating rather than an individual boiler.
Check which system serves your home before booking a private engineer, so you don’t pay for a repair that’s the building’s responsibility.
The wet side — radiators, valves and pumps — can be done by a competent plumber.
Work on the boiler itself and the gas supply must be done by a Gas Safe registered engineer.
Always check the ID card covers boiler work.
Usually scale or sludge on the heat exchanger causing water to boil locally.
It signals reduced efficiency.
It is normally solved by cleaning and water treatment — not necessarily a new boiler.
Central heating repair across Greenwich — areas we cover
Central Heating Repair Eltham · New Eltham · Mottingham · Kidbrooke · Blackheath · Greenwich · Charlton · Woolwich · Plumstead
Related services in Eltham
Boiler Repair · Boiler Installation · Leak Detection · Emergency Plumber
Related guides
Combi vs System Boiler · Boiler Fault Codes · Boiler Repair or Replace · London Hard Water Guide · London Plumbing Costs Guide
When the heating fails in an Eltham home, the boiler is only one suspect — sludge, air, a stuck valve, a tired expansion vessel or a slow leak account for most of it. The value is in the diagnosis: find where the heat is actually being lost, treat the system water, and fix the cause rather than the symptom.
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Source provenance
Regulatory and safety guidance on this page is drawn from primary UK sources: the Health and Safety Executive (Gas Safe registration required for domestic gas work; do not use a gas appliance you suspect is unsafe; the gas-leak procedure and the National Gas Emergency number 0800 111 999), Thames Water (all the water in its region is hard), and the Royal Borough of Greenwich (council repair timescales for loss of heating; council-tenant repair and gas-problem routing; leaseholder responsibility for internal heating systems; private-rented escalation). Cost figures are an editorial estimate only — not regulated rates and not official market data — and Eltham housing observations are local editorial context.
Sources & further reading
- HSE — Gas safety: home owners (Gas Safe registration required for domestic gas work; don’t use an appliance you suspect is unsafe; gas-leak procedure; 0800 111 999) — https://www.hse.gov.uk/gas/domestic/faqownerocc.htm
- Thames Water — Hard water (“all the water in our region is hard, because it passes through chalky limestone”) — https://www.thameswater.co.uk/help/water-and-waste-help/water-quality/hard-water
- Royal Borough of Greenwich — How long it takes to repair a problem (heating restored within 1 working day, 1 Nov–30 Apr; 3 working days, 1 May–31 Oct) — https://www.royalgreenwich.gov.uk/housing/request-repair/how-long-it-takes-repair-problem
- Royal Borough of Greenwich — Request a repair to your council home (020 8921 8900 in office hours; 020 8854 8888 at all other times) — https://www.royalgreenwich.gov.uk/housing/request-repair/request-repair-your-council-home
- Royal Borough of Greenwich — Check what’s happening with your repair (gas: Building Services 0800 317 715 or K&T Heating 0800 0434 726) — https://www.royalgreenwich.gov.uk/housing/request-repair/check-whats-happening-your-repair
- Royal Borough of Greenwich — Who can repair your problem if you’re a leaseholder (leaseholders responsible for heating and hot-water systems inside the home) — https://www.royalgreenwich.gov.uk/housing/request-repair/request-repair-your-leasehold-home/who-can-repair
- Royal Borough of Greenwich — Report a problem with a private rented home (Residential Services 020 8921 8157; 14-day landlord response) — https://www.royalgreenwich.gov.uk/housing/request-repair/report-problem-private-rented-home
Last reviewed: July 2026 by Adiel Khan — SFEDI-accredited business advisor, 20+ years’ experience (South East Enterprise Ltd) and operator of Verified Plumbers. LinkedIn ↗ This page is checked for compliance and regulatory accuracy against HSE, Gas Safe Register, GOV.UK legislation, Thames Water and Royal Borough of Greenwich guidance. Source links are provided within this page where relevant.